The present invention relates to a device and method for controlling output from a rechargeable battery.
In recent years, rechargeable batteries have been combined with fuel cells, solar cells, or power generators to form power supply systems. A power generator is driven by natural power, such as wind power or water power, or by artificial power, such as power generated by an internal combustion engine. A power supply system using a rechargeable battery stores excess power in the rechargeable battery to improve energy efficiency.
An example of a power supply system is a hybrid electric vehicle (HEV) using an engine and a motor as its power sources. When the engine outputs more power than necessary to drive the vehicle, the HEV drives its generator using the excessive power to charge the rechargeable battery. When the vehicle is braking or decelerating, the HEV drives the motor with the vehicle wheels and charges the rechargeable battery using the motor as a power generator. When the output from the engine is small, the HEV compensates for the lack of power by discharging the rechargeable battery and driving the motor.
In this way, the HEV accumulates energy in the rechargeable battery. Conventional automobiles release such energy into the atmosphere as heat. The energy efficiency of an HEV is higher than the energy efficiency of a conventional automobile. Thus, the HEV greatly improves fuel efficiency as compared with conventional automobiles.
To efficiently charge excess power into the rechargeable battery of the HEV, the rechargeable battery is controlled in a manner that its state of charge (SOC) does not reach 100%. To drive the motor when necessary, the rechargeable battery is also controlled in a manner that its SOC does not reach zero. More specifically, the rechargeable battery is normally controlled in a manner that the SOC varies within a range of 20 to 80%.
A rechargeable battery installed in the HEV or other power supply systems may be formed by connecting a plurality of battery cells (cells) in series. In this case, the rechargeable battery may be overdischarged due to variations in the capacities of the battery cells. When a battery cell having a small capacity is overdischarged, a polarity reversal may occur. This shortens the life of the rechargeable battery.
To prevent such a polarity reversal from occurring, output from the rechargeable battery may be restricted when the terminal voltage of the rechargeable battery falls and becomes equal to a predetermined value or less. For example, Japanese Laid-Open Patent Publication No. 2003-199258 (FIGS. 3 and 4) describe a controller (hereafter referred to as a “battery ECU”) for controlling output from a rechargeable battery. The battery ECU sets a maximum value to limit the discharge power that can be output from the rechargeable battery. Then, the battery ECU transmits the set output maximum value to a vehicle controller (hereafter referred to as a “vehicle ECU”). As a result, when driving the motor, the vehicle ECU drives restricts the output from the rechargeable battery so that the output does not exceed the maximum value.
The vehicle ECU requests the rechargeable battery for output that differs in accordance with the conditions of the vehicle. For example, in comparison to when the vehicle is being driving in a stable state, the vehicle ECU requests the rechargeable battery for a higher output within a shorter period of time when the vehicle starts to move or when a gear is changed. Therefore, if the maximum output value were to be set based on a stable traveling state, high output during a short period of time would not be allowed. This increases the load on the engine.
Accordingly, in the above publication, the battery ECU sets two types of output maximum values. Further, in addition to the process for restricting output from the rechargeable battery, the battery ECU performs processes, such as the calculation of the state of charge (SOC), output of the calculated SOC, determination of deterioration of the rechargeable battery, and output of the determination result.
FIG. 1 is a timing chart showing the control of output from the rechargeable battery. In FIG. 1, the vertical axis represents the terminal voltage Vb and output maximum values Pp and Pn (maximum values of discharge power) and the horizontal axis represents time.
As shown in FIG. 1, the battery ECU (not shown) sets the short-time output maximum value Pp and a long-time output maximum value Pn. The short-time output maximum value Pp represents the maximum value of the discharge power when the vehicle requests for a high output during a short period of time (for example, two seconds). The long-time output maximum value Pn represents the maximum value of the discharge power when the vehicle is being driving in a stable state. In a normal state, the short-time output maximum value Pp is set at 28 kW and the long-time output maximum value Pn is set at 20 kW. That is, the short-time output maximum value Pp is greater than the long-time output maximum value Pn. This enables the vehicle ECU to drive the motor when the vehicle starts to move or when a gear is changed.
In the example of FIG. 1, the battery ECU lowers the short-time output maximum value Pp and the long-time output maximum value Pn to prevent polarity reversal of the rechargeable battery. More specifically, the battery ECU lowers the long-time output maximum value Pn when the terminal voltage Vb decreases to, for example, 12 V. The lowering of the long-time output maximum value Pn is performed in a graded manner in accordance with decrease in the terminal voltage Vb. Further, the battery ECU also lowers the short-time output maximum value Pp.
The lowering of the short-time output maximum value Pp is first performed when the terminal voltage Vb decreases to, for example, 9.6 V. At the same time, the short-time output maximum value Pp is lowered to be substantially the same as the long-time output maximum value Pn. Further, even if the terminal voltage Vb increases again and exceeds 12 V, the battery ECU lowers the short-time output maximum value Pp and the long-time output maximum value Pn if the terminal voltage Vb decreases again to 12.0 V during a certain period.
The two types of output maximum values are set in this manner in the example of FIG. 1. Thus, the motor is effectively driven when the vehicle starts to move or when a gear is changed. This also prevents a polarity reversal from occurring.
However, in the example of FIG. 1, the terminal voltage at which the lowering of the short-time output maximum value Pp is started is lower than the terminal voltage at which the lowering of the long-time output maximum value Pn is started. Thus, when the lowering of the short-time output maximum value Pp starts, the difference between the short-time output maximum value Pp and the long-time output maximum value Pn increases.
Thus, in the example of FIG. 1, the discharge power of the rechargeable battery may suddenly be restricted when the output request to the battery ECU is shifted from high output to normal output within a short period of time. This lowers drivability of an HEV.